*{
    margin: 0px;
    padding: 0px;
}
html{
    display: flex;
    justify-content: center;
    height: 100%;
}

.big1{
    display: flex;
    margin-top: 50%;
    width: 360px;
    height: 100px;
}
.small1{
    width: 120px;
    height: 100px;
    background-color: red;
}
.small2{
    width: 120px;
    height: 100px;
    background-color: green;
}
.small3{
    width: 120px;
    height: 100px;
    background-color: blue;
}